Site-specific controls predominate over erosion effects on nitrogen turnover: Evidence from 15N tracing in the field

Understanding how nitrogen (N) cycling is affected by soil erosion is essential to optimize N supply for crops and reduce losses in hilly fields. While topography-related impacts on N availability and losses have been studied, the mechanisms linking erosion to N transformation dynamics remain unclear. We conducted a field experiment at two sites to investigate gross N transformation, plant N uptake, and nitrous oxide (N 2 O) emissions from different sources in erosion-affected soils. Two 15 N-labelling approaches were combined: process-based modelling of N transformation following in situ 15 N tracing after fertilization and season-long 15 N balancing, to trace 15 N-enriched fertilizer N in maize biomass, soil and N 2 O emissions. Although differences in gross N transformation rates, N 2 O emissions, crop yields, and total 15 N recovery were more pronounced between sites than between soils, several N measures showed site-specific trends across erosion-affected soils. In Dedelow, gross N mineralization, NH 4 + immobilization, and NH 4 + oxidation were at least twofold higher in eroded compared to non-eroded and deposited soils. This contrasts with the expectation that eroded soils generally exhibit reduced N turnover and the lowest soil fertility. In Friedrichshof, seasonal N 2 O emissions tended to be higher in deposited and eroded soils than non-eroded soils. However, N measures did not follow the same pattern across soils between sites, except for crop yield and soil-derived N supply, both increasing in the order non-eroded < eroded < deposited soils at both sites. Our findings emphasize the importance of temporal and site-specific conditions for managing N supply and losses in erosion-affected soils.